The continuing trend of scaling down integrated circuits has motivated the semiconductor industry to consider new techniques for fabricating precise components at sub-micron levels. As is the case for most semiconductor integrated circuitry, circuit density is continuing to increase at a fairly constant rate and a major makeup of many integrated circuits is the field effect transistor (FET). The typical FET structure is formed in a silicon substrate, with the source/drain implanted into the horizontal substrate surface, the channel spanning there between and the gate formed over the channel. A second FET structure is a vertical oriented transistor, such as a vertical-sided-gate field effect transistor (VSGFET).
The VSGFET structure is oriented such that the source/drain and channel of the transistor are formed vertically in a silicon substrate by forming vertical silicon pillars in the silicon substrate, while the gate wraps around the channel region of the vertical pillars. A key aspect in forming the VSGFET is in the definition of the gate length. One fabrication approach to define the gate length of a VSGFET is depicted in FIG. 1-5.
The overhead view of FIG. 1 shows a series of circular nitride hard masks 11 defining columns of vertical silicon pillars separated by shallow trench isolation 12. A cross-section taken through line 1-1′ of FIG. 1 is depicted in FIG. 2. As seen in FIG. 2, the vertical silicon pillars 20 are formed by etching into the silicon substrate 10 by using the nitride hard mask 11 as an etching guide. Shallow trench isolation 12 is formed between each column of silicon pillars.
As shown in FIG. 3, a conformal gate dielectric 30 is formed on the substrate surface such that it coats the horizontal surface of silicon substrate 10, the shallow trench isolation 12, the vertical sidewalls of the silicon pillars 20 and the nitride hard mask 11.
As shown in FIG. 4, a polysilicon 40 is deposited to fill the spaces between the silicon pillars 20. Then the polysilicon 40 is planarized along with a top portion of the nitride hard mask 11.
As shown in FIG. 5, the polysilicon 40 is recessed to a designed thickness, which will expose an upper portion of the gate dielectric 30 as well as define the gate channel length of the vertical gated transistor. This approach has two main potential problems in that the recessing of polysilicon 40, typically by a plasma etch, has the tendency to damage the gate dielectric/polysilicon interface and the plasma etch causes unavoidable round corners 50 above the major horizontal surface of the vertical-surrounding-gate at the gate dielectric/polysilicon interface. These rounded corners 50 will increase gate channel length variation across the silicon substrate 10. Furthermore any misalignment between the gate polysilicon pattern and the silicon pillar 20 will increase the serial resistance of each transistor structure along with potential gate damage due to exposing the silicon channel.
The present invention describes a vertical-surrounding gate field effect transistor formed by a method to define a gate channel length for a vertical-surrounding gate field effect transistor with self-aligning features that addresses the above challenges, the method disclosed herein for use in the manufacture of semiconductor devices or assemblies, which will become apparent to those skilled in the art from the following disclosure.